Integrated magnetic and elastic force systems

Abstract

Magnetic force increases as the distance (d) of the force- generating elements (F ~ 1/d2) decreases, whereas elastic force decreases as the distance decreases (F ~ kd). These opposing characteristics suggest that combining both force systems will establish an integrated system with a long-range working ability. The objective of this study was to determine the vertical closure force (FX) and the transverse axis moment (MY) of an integrated force system, ie, attracting magnets with elastics (vertical or Classes II and III). FX and MY were examined on the orthodontic measurement and simulation system. It was found that the integrated force system had a positive closure force (+FX) that never declined to 0 and a long working range. Three regions characterized the force-deflection curve of FX: the magnetic region (0-3 mm, for magnets with 3/16-in medium elastics), in which the decline in magnetic force was larger than the increase in elastic force (6.3-2.5 N); the constant region (3-7 mm), in which the decline in magnetic force equaled the increase in elastic force (2.5-2.9 N); and the elastic region (7-10 mm), in which there was only an increase in elastic force (2.9-3.5 N). The transverse axis moment (+MY), which tends to close the bite, developed especially in magnets with a single vertical elastic. Clinically, inactivation of vertical elastics by closing the mouth can be overruled by the integrated force system because it exerts adequate force level at both short and long distances. (Am J Orthod Dentofacial Orthop 2002;122:155-63)